This invention relates to an item with external teeth, such as a gear, and a method of forming the item.
Conventionally, formation of an item with external teeth such as a gear is implemented by a method of forming teeth by cutting a workpiece such as a forged blank into shape or a method of forming teeth by forging a workpiece into shape. In the former method of teeth formation by cutting, however, the working speed is too low and a grinding process is required for improving the surface roughness of the teeth after the cutting process. Accordingly, the former method involves much time taken for teeth formation. On the other hand, the latter method of teeth formation by forging causes problems of enormous expense in plant and equipment, difficulty in fabricating a die and deterioration in work environment due to the use of lubricant, for example, in a Bonderite process, for the purpose of increase in die life and formability.
Meanwhile, as disclosed in the specification and drawings of German Patent Publication No. 19613457, there is a method of generating external teeth in the outer periphery of a workpiece with the use of an external gear tooth generating device including a form roller having an outer periphery formed with four gear-like tooth generating sections of different tooth heights changed in four steps along the rotation axis of the roller to increase the outer diameter of the roller in a stepwise manner. In this method, the tooth generating sections are pressed in increasing order of tooth height against the outer periphery of a rotating workpiece so that the rotation force of the workpiece is transmitted to the form roller. The form roller thereby step wise generates a plurality of teeth of given shape in the outer periphery of the workpiece while rolling therealong. According to this method of external teeth generation, since the teeth are generated by plastic deformation of a workpiece, the drawbacks due to cutting as described above cannot be caused. In addition, since the form roller can be made relatively cheap and easily, the problems due to forging as described above never occur. It is to be noted that the reason why the generation of teeth of given shape (tooth grooves of given depth) is made stepwise in four divided generating steps is that teeth generation in a single step would create extended or recessed tooth portion thereby not providing a complete tooth profile.
Furthermore, in the above method of external teeth generation, since both the workpiece and the form roller are rolled one along another to generate the external teeth, it is difficult that the tooth generating sections of the form roller are pressed evenly against both sides of each tooth which serve as mating surfaces of each of fully generated teeth. As a result, the mating surfaces of each of the teeth easily produce a variation in forming accuracy. This is a serious problem for gears requiring high accuracy. In order to eliminate such a variation, the teeth must be finished by cutting. In this case, however, the flow line of metal texture having been formed along the tooth profile during the tooth generating process is broken, resulting in decreased strength.